A wide variety of electronic systems and subsystems, such as, but not limited to those employed in airborne and spaceborne signal collection and processing applications, require that their electronic circuits and components of such circuits be housing in packaging structures that are lightweight, have the capability of efficiently dissipating heat, and can be hermetically sealed against the surrounding environment. One example of a conventional packaging structure designed for this purpose is disclosed in the U.S. patent to Jones et al, No. 6,355,362 (hereinafter referred to as the '362 patent).
As described therein, and as diagrammatically illustrated in the respective plan and cross-sectional views of FIGS. 1 and 2, which respectively correspond to FIGS. 7A and 7B of the '362 patent, the electronics packaging structure of the prior art comprises a generally rectangular-configured body 10 having a hollow interior space, that is defined by a bottom interior floor 12 and adjacent sidewalls 14-17. The package body 10 is made of a material having a relatively low coefficient of thermal expansion, so that it may be readily hermetically sealed and generally comply with intended physical parameters of the support assembly to which it is mounted. To this end, the package body 10 is typically made of titanium or an alloy thereof. Unfortunately, titanium has a relatively poor thermal conductivity relative to that of other materials, such as aluminum (which cannot be used due to its relatively high CTE), so that it will not readily allow the removal therethrough of heat generated by the electronic circuit components to be retained within the interior space of package body 10.
In an effort to accommodate the low thermal conductivity of titanium, the bottom floor 14 of the prior art packaging structure according to the '362 patent is selectively penetrated by a plurality of bores or apertures (four of which are shown). These bores are selectively placed at locations of the floor 14 where circuit components are to be installed, and have their internal volumes sized to accommodate a minimum amount of a high thermal conductivity, low CTE, secondary material, that is able to remove sufficient heat for maintaining proper operation of circuit components mounted thereon, while minimizing the mass thereof. Filling these bores with such secondary material forms respective intrusive heat sink regions 20A-20D, that penetrate the bottom floor 12 of the package body. Placement of a heat removal member, such as a cold plate, against the bottom of the package 10 to which the heat sink regions 20A-20D extend allows heat to be removed from circuit components mounted thereon. (In some conventional packages, rather than directly abut a respective circuit component against its associated heat sink region, the bottom of the circuit component may be mounted against an additional heat sink element which, in turn, is mechanically attached to the heat sink region by means of a thermally conductive epoxy adhesive. Unfortunately, because thermally conductive epoxy contains dielectric material, its thermal conductivity is less than that provided by a purely metal interface.)
In addition to having a relatively high thermal conductivity (in order to cool the circuit components), the secondary material of the heat sink regions 20A-20D has a relatively low CTE proximate to that of the (titanium) package body 10, so that the heat sink regions may be hermetically sealed with the bottom of the package body 10. Secondary materials listed in the '362 patent include molybdenum alloys, such as copper-molybdenum alloy (whose component percentages may be defined so that its CTE matches the CTE of the titanium body), and aluminum-silicon-carbide (AlSiC), whose CTE is proximate to (slightly less than) that of the titanium body 10.
A major shortcoming of the architecture of the prior art electronics package structure disclosed in the '362 patent is the fact that it is process-intensive, and therefore costly, since formation of the heat sink bores in the bottom floor of the package body involves a precision machining tolerance operation, in order to accurately form the bores at the exact locations where the circuit components are installed. In addition, for optimal hermetic sealing (minimal mechanical stress at the hermetic joints) of the heat sink regions with the titanium housing, it is preferred that the secondary material of the heat sink regions have a CTE that effectively matches that of the titanium body. Although this can be accomplished by employing copper-molybdenum alloy as the secondary material, the use of such an alloy entails a substantial weight penalty, as the weight of Cu—Mo alloy is on the order of three times the weight of titanium.